You're right about the extreme angle. In fact the angle is so extreme that it won't work unless you also make the horn a little narrower. That will give you a little more room to work with. If you just leave it, the angle gets so extreme that the drivers will bottom out and won't fit. I know I've said before that I will take pics of mine, but this time I mean it.

Here's the pics of the WO 15. The baffles are somewhat removable at this point due to the fact that you can't get the speakers mounted with them in place. I still need to come up with a good way of mounting the baffles and keep the ability to remove them.

Your two examples may be on par with eachother, then again may not, there are alot of factors to think about.

Why do people go for 15's though? Simply to hit lower frequencies and/or have more usable bass in those ranges. Why is this possible? It's just down to 15's having a substantially larger cone area than 10's or 12's.